A typical biopotential amplifier system includes an amplifier module connected to a patient headbox with a multi-conductor cable. Patient electrodes are connected between a patient and the headbox. A typical amplifier has multiple electrode inputs or channels, for example, 8, 16, 32, or 64 channels.
Common mode rejection ratio (CMRR) is one measurement of an amplifier's performance. CMRR indicates the ability of an amplifier to reject common mode interference, typically 50 or 60 Hz, depending upon the power source, e.g., AC power. Common mode voltage can be reduced by driving an inverted version of the patient common-mode signal back into the patient in a negative feedback loop, commonly called the right leg drive (RLD). In this way right leg drive effectively increase the CMRR of a biopotential amplifier system.
FIG. 1 shows a conventional cable 100 for use with a patient headbox for acquiring biopotential measurements having a bundle of wires surrounded by a shield 110, which is itself surrounded by an outer jacket 120. This bundle includes the multiple channel (e.g., patient) electrode wires 130, a reference electrode wire 140, and a right leg drive (RLD) electrode wire 150.
This conventional configuration has drawbacks in that the achievable CMRR is lower then possible. This aforementioned low CMRR results from capacitance, e.g., parasitic capacitance, between the RLD wire 150 and the channel electrode wires 140 due to the close proximity between them in the cable 100. Moreover, this capacitance allows coupling of the RLD signal to the channel wires 130 bypassing the patient. Unbalance of this parasitic capacitance works in conjunction with the patient electrode impedances to reduce the CMRR of the amplifier system. The higher the patient electrode impedance the larger the potential difference between the patient and the channel wires.
Accordingly, there is a need and desire to provide a cable with reduced coupling between the RLD and channel wires for enhancing biopotential measurements and increasing the CMRR of a biopotential amplifier system.